1. Field of the Invention
The present invention relates to a bearing structure for motors in which rolling bearings are used.
2. Statement of the Prior Art
Where it is required to accurately position a spindle that is an output shaft of a motor in a thrust direction and rotatably support said spindle while preventing its rattling, as encountered in the case of, e.g., a direct drive system for disks that are recording media for information carrying signals, the spindle is supported by rolling bearings, while the rolling bearings are biased by a spring in the thrust direction to apply a pilot pressure thereto.
Typically, FIGS. 8 and 9 show two conventional bearing structures for motors in which a pilot pressure is applied in the thrust direction of rolling bearings.
Referring to FIG. 8, a spindle hub 1a is fixed to an upper end of a spindle 1 that is an output shaft of a motor, and inner rings of two rolling bearings 2 and 2 are fixed around upper and lower portions of the spindle 1 with a sleeve 3 interposed therebetween. At a lower end of the spindle 1, there is fixed a rotor case of the motor by means of a mounting screw 6. An outer ring of the upper bearing 2 is fixed into a central hole in a housing 9, whereas an outer ring of the lower bearing 2 is fitted into the central hole in the housing for relative movement in the axial direction. Between the outer ring of said lower bearing 2 and a step of the housing 9, there is interposed a pilot pressurization spring 4 for applying downward and upward biases to the outer ring of said lower bearing 2 and the housing 9, respectively.
The bearing structure shown in FIG. 8 is called a constant-pressure pilot pressurization system wherein the pilot pressurization spring 4 fluctuates depending upon the relative movement between the outer ring of the lower bearing 2 and the housing 9 so that a constant pilot pressure is always obtained.
In contrast to the conventional bearing structure shown in FIG. 8, such a sleeve 3 as employed therein is removed from the conventional bearing structure shown in FIG. 9. As illustrated in FIG. 9, an outer ring of a lower bearing 2 is fixed into a central hole in a housing 9, and between a rotor case 8 of a motor fixed to a lower end of a spindle 1 by means of a screw 6 and an inner ring of a lower bearing 2 there is interposed a pilot pressurization spring 7 for applying downward and upward biases to the rotor case 8 and the inner ring of the lower bearing 2, respectively.
The conventional bearing structure shown in FIG. 9 is referred to as a constant-position pilot pressurization system wherein the pilot pressurization spring 7, once positioned, is restrained from vertical fluctuation.
A problem with the bearing structure shown in FIG. 8 is that since the inner rings of the upper and lower bearings are fixed in place through the sleeve located on the inside of said inner rings, the accuracy of the sleeve has a considerable influence upon axial or planar vibrations.
In the bearing structure shown in FIG. 9, a central constricted portion of the rotor case is located in the inner rings of the bearings to determine the central position of the rotor case, so that when mounting the rotor case in place by means of a mounting screw, said constricted portion of the rotor case tends to come in contact with the inner rings of the bearings. When the constant-pressure pilot pressurization system is applied to this bearing structure, the resulting pilot pressurization tends to be unsatisfactory due to a frictional force occurring between said constricted portion of the rotor case and the inner rings of the bearings. This is the reason that applied to the bearing structure of FIG. 9 is the constant-position pilot pressurization system wherein both the inner and outer rings of the upper and lower bearings are fixed to the spindle and housing. According to this pilot pressurization system, the inner rings of the bearings have to be fixed to the outer rings thereof under a certain pilot pressure, and difficulty is encountered in providing a confirmation of whether or not the certain pilot pressure is applied, once the inner rings of the bearings have been fixed to the outer rings thereof. This structure is also lacking in reliability, since the pilot pressure is maintained by the bonding of the inner or outer rings of the bearings. For that reason, there is proposed a structure wherein a pilot pressurization member is located in opposition to the inner rings of bearings and an axial push is given to said pilot pressurization member with a jig to apply a pilot pressure to the bearings, whereupon said pilot pressurization member is fixed to a spindle with the use of a screw. However, this structure is troublesome to assemble and regulate, costs much and is disadvantageous for making the bearings thin.